Use of dither-quantizing with stationary dithers to reproduce photographs by means of coarsely-quantized dot samples is well known. A black-and-white picture, for example, is reproduced with a stippled texture correpsonding to oscillations between two adjacent quantum levels of the gray scale, and the psychovisual system performs low-pass filtering of this texture to give the human observer the impression of intermediate grays. Ordered dithers can be designed to minimize the visibility of the stippled textures while also preserving pictorial information which the same gray-scale quantizing would otherwise have destroyed. These ordered-dither patterns have also been used in stationary dithers for television signals; reference [5] teaches the use of a particular variety, based on nasik dither matrices, that, unlike other well-known and efficient ordered dithers, can be oriented to avoid large-area flicker on TV displays having the usual 2:1 lines interlace. Nasik dithers and similar optimal ordered dithers of prior art always have a number of dither sizes that is an integral power of 2.
The perceived picture quality suffers, in respect to both transmitted information and the visibility of stipple, if the dither samples of a stationary ordered dither pattern are redistributed in random fashion. However, processing at a TV receiver to subtract the identical random frame pattern from a received signal (following Roberts, U.S. Pat. No. 3,244,808) inserts additional grays between the quantum levels in a manner that reduces the r.m.s. error by half, improving the random-dithered picture. Optimal ordered dithers neither require nor benefit from such subtraction and provide equal or better picture quality with a much simpler system. Roberts generates stationary random dither by means of a pseudo-random sequence generator that is reinitialized for each frame, insuring a fixed frame pattern; prolonging the sequence to provide 3-d random dither with uncorrelated frame patterns would impair the picture quality.
Prior-art 3-d television dithering is found in Thompson and Sparkes ("A Pseudo-Random Quantizer for Television Signals"; Proc. IEEE, vol. 55, no. 3, March 1967) and in References [1]-[5]. The system of Thompson and Sparkes is similar to that of Roberts, adding a dither before quantizing and subtracting the same dither at the receiver, except that the former combine Roberts' stationary dither with a 2-phase cinematic dither comprising only two sizes arranged according to a checkerboard frame pattern that reverses every other frame. Reference [5] describes cinematic dithers having either eight or sixteen sizes and a number of phases that is an integral power of 2. The frame patterns are nasik-type ordered-dither patterns and cycling of the sizes on a pattern element is achieved by negating bits in the binary numbers representing the eight or sixteen sizes. This causes all sizes to cycle according to one sequence on half of the elements and in reverse order on the remaining elements. Generation of such cinematic nasik dithers is also described in Reference [ 1], [2] and [3]. Nasik frame patterns are typical of of ordered-dither patterns having the number of dither sizes equal to an integral power of 2 for optimum dither-quantizing of still pictures. Efficient cinematic dithers construction from such frame patterns need a number of phases that is also a power of 2.
Reference [5] discloses the use of cinematic dithering in a monochrome TV system, and References [1] through [4] disclose their use in color-television systems. Dither-quantizing reduces the number of bits per sample needed in digital television signals; and in other systems, including NTSC-compatible systems, it makes signal regeneration possible and facilitates encrypted transmission.